Conventionally, in a thin film magnetic recording medium, a lubricating agent is applied onto a surface of a magnetic layer in order to reduce friction and wear between a magnetic head and a surface of the medium. An actual film thickness of the lubricating agent is at the molecular level in order to avoid adhesion such as stiction. Therefore, it's no exaggeration to say that the most important thing in the thin film magnetic recording medium is to select a lubricating agent having excellent wear resistance under every environment.
It is important to allow the lubricating agent remain on the surface of the medium without detachment, spin-off, or chemical deterioration throughout a service life of the magnetic recording medium. The more difficult it is to allow the lubricating agent remain on the surface of the medium, the smoother the surface of the thin film magnetic recording medium is. This is because the thin film magnetic recording medium has no ability for replenishing the lubricating agent, unlike a coating type magnetic recording medium.
In the case where the lubricating agent is only weakly adhered to a protecting film on the surface of the magnetic layer, a large quantity of lubricating agent is required because the film thickness of the lubricating agent is decreased upon heating or sliding to thereby accelerate wear. The large quantity of lubricating agent results in a migrating lubricating agent which can have the ability for replenishing the lubricating agent removed through wear. However, there is a dilemma that an excess of lubricating agent causes the film thickness of the lubricating agent to be greater than surface roughness to thereby cause a problem related to adhesion, and eventually the stiction contributing to drive failure, which is fatal. The problem related to friction has not been satisfyingly solved by conventional perfluoropolyether (PFPE) based lubricating agents.
Particularly, in a thin film magnetic recording medium having high surface smoothness, a novel lubricating agent has been molecularly designed and synthesized in order to solve the trade-off. Many reports on a lubricating property of PFPE have been submitted. Thus, the lubricating agent is very important for the magnetic recording medium.
Table 1 shows chemical structures of representative PFPE based lubricating agents.
TABLE 1Fomblin based lubricating agentsX—CF2(OCF2CF2)n(OCF2)mOCF2—X (0.5 < n/m < 1)ZX = —OCF3Z-DOLX = —CH2OHZ-DIACX = —COOHZ-TetraolX = —CH2 OCH2CHCH2OH    OH AM2001 Other lubricating agentsA20H MonoF—(CF2CF2C2O)1—CF2CF2CH2—N(C3H7)2
Z-DOL in Table 1 is one of commonly used lubricating agents for thin film magnetic recording media. Z-tetraol (ZTMD) is those in which a functional hydroxyl group is additionally introduced in a main chain of PFPE, which has been reported to enhance reliability of the drive while reducing gap of a head-media interface. A20H has been reported to prevent the main chain of PFPE from being decomposed by a Lewis acid or a Lewis base, and improve tribological characteristics. Meanwhile, Mono has a different polymeric main chain and polar group from the above described PFPE, that is, polynormal propyloxy and amine, respectively; and has been reported to decrease adhesive interactions in near contact.
However, a common solid lubricating agent which is believed to have a high melting point and thermal stability interferes with highly sensitive electromagnetic conversion process and deteriorates wear characteristics due to wear debris produced in a running track when the agent is scraped by the head. The liquid lubricating agent as described above has a migrating property which is an ability in which a lubricating agent removed through wear is replenished with a lubricating agent migrated from the adjacent lubricating layer. However, due to the migrating property, the lubricating agent is decreased by spin-off from the surface of the disk during disk operation especially under a high temperature, so that a protective function is lost. Therefore, a lubricating agent having high viscosity and low volatility has been suitably used, which allows for low evaporation rate and long service life of the disk drive.
In view of these lubricating mechanisms, a low friction and low wear lubricating agent used in the thin film magnetic recording medium is required to have the following requirements:
(1) low volatility;
(2) low surface tension for surface replenishing ability;
(3) interaction between a terminal polar group and a surface of a disk;
(4) high thermal and oxidative stability to prevent decomposition or a decrease during use;
(5) chemically inert to metal, glass, and polymer, and no wear debris produced by a head or a guide;
(6) no toxicity or flammability;
(7) excellent boundary lubricating property; and
(8) solubility in organic solvents.
Recently, an ionic liquid has been attracting attention as one of environmental friendly solvents for synthesizing organic or inorganic materials in electricity storage materials, separation technologies, and catalyst technologies. The ionic liquid is broadly categorized into a molten salt having a low melting point, and generally refers to the molten salt having the melting point of 100° C. or lower. Important properties of the ionic liquid used as the lubricating agent include low volatility, no flammability, thermal stability, and excellent solubility. Therefore, the ionic liquid is expected to be applied as a novel lubricating agent under an extreme environment such as in vacuum or in a high temperature due to its characteristic. It has also been known that use of the ionic liquid in a gate of a single self-assembled quantum dot transistor improves the controllability of the transistor by a factor of one hundred over a conventional one. In this technique, the ionic liquid forms an electric double layer and serves as an about 1 nm-thick insulating film to thereby obtain a large electric capacity.
For example, use of a certain ionic liquid may reduce friction and wear on a metal or ceramic surface in comparison to a conventional hydrocarbon based lubricating agent. For example, it has been reported that, in the case where an imidazole cation based ionic liquid is synthesized by replacing with a fluoroalkyl group, and alkylimidazolium tetrafluoroborate or hexafluorophosphate is used on steel, aluminium, copper, single crystal SiO2, silicon, or sialon ceramics (Si—Al—O—N), it shows more excellent tribological characteristics than cyclic phosphazene (X-1P) or PFPE. It has also been reported that an ammonium based ionic liquid reduces friction in from elastohydrodynamic to boundary lubrication region compared with a base oil. In addition, an ionic liquid has examined for an effect as an additive for the base oil, or researched on a chemical and tribological chemical reaction in order to understand its lubricating mechanism. However, there are few application examples as the magnetic recording medium.
Among ionic liquids, a protic ionic liquid is a generic designation of compounds formed through a chemical reaction between a Bronsted acid and a Bronsted base in equal amounts. Research by Kohler et al. related to an interaction between carboxylic acid and amine has been reported a 1:1 complex of carboxylic acid and amine in chemically equal amounts can be formed (e.g., see NPLs 1 and 2). Perfluorooctanoic acid alkyl ammonium salt is the protic ionic liquid (PIL), and has been reported to have an effect of reducing friction in the magnetic recording medium significantly higher than the above described Z-DOL (see, PTLs 1 and 2, and NPLs 3 to 5).
However, these perfluorocarboxylic acid ammonium salts have a weak interaction between a cation and an anion in the reaction shown by the following Reaction scheme (A). Therefore, the equilibrium is shifted towards left side under a high temperature in accordance with Le Chatelier's law to thereby produce dissociated neutral compounds, leading to thermal instability. That is, proton transfer occurs under the high temperature and the equilibrium is shifted towards the neutral substance to thereby dissociate (e.g., see NPL 6).CnF2n+1COOH+CnF2n+1NH2⇄CnF2n+1COO−H3N+CnH2n+1  (A)
The limit of a surface recording density of a hard disk is said to be 1 Tb/in2 to 2.5 Tb/in2. At present, the limit is being approached, but energetic development has been continued for increasing a capacity on the assumption of refining magnetic particles. The technologies for increasing the capacity include decreasing effective flying height or introducing Shingle Write (BMP).
Additionally, “Heat Assisted Magnetic Recording” is known as a next-generation recording technology. FIG. 3 is a schematic view illustrating the heat assisted magnetic recording. In the technology, a recording portion is heated with laser upon recording/reproduction, and therefore, there is a problem that durability is deteriorated due to evaporation or decomposition of the lubricating agent on a surface of the magnetic layer. In the heat assisted magnetic recording, the magnetic recording medium may be exposed to a high temperature which is said to be 400° C. or higher even in a short time. Therefore, commonly used lubricating agents for the thin film magnetic recording media, Z-DOL and a carboxylic acid ammonium salt based lubricating agent, are concerned about their thermal stability.
The ionic liquid is generally a substance having high thermal stability because it forms ions as described above. The equilibrium thereof is shown in the following Scheme 1.

In the above Scheme, HA denotes a Bronsted acid, and B denotes a Bronsted base. The acid (HA) and the base (B) react with each other as shown in Scheme 1 to thereby form a salt (A−HB+).
Herein, the dissociation constants Ka1 and Kb2 of the acid and the base can be shown by the following Scheme 2 using concentrations thereof.

The Ka1 and the Kb2 vary widely depending on substances, and, in some cases, they have the large number of digits. However, the large number of digits is inconvenient for handling, so that it is often expressed as the negative common logarithm. That is, as shown in the following Scheme 3, it is defined that −log10 Ka1 is equal to pKa1. Obviously, the smaller pKa1 is, the stronger the acidity is.
Next, the difference between the dissociation constants of the acid and the base, ΔpKa, will be discussed. A reaction between an acid and a base is influenced by their acidity or basicity (or acidity of a conjugate acid), and the difference of acidity ΔpKa can be shown by the following Scheme 3.

As can be seen from the above Scheme, the ΔpKa becomes larger as the salt concentration [A−HB+] becomes larger relative to the acid and base concentrations.
In particular, Yoshizawa et al. have been reported that, when the difference between the pKa value of the acid and the pKa value of the base (ΔpKa) is 10 or more, the proton transfer is more likely to occur, and[AH]+[B]⇄[A−HB+]the equilibrium is shifted towards an ion side (right side) to thereby enhance stability (e.g., see NPL 6). However, MacFarlane et al. have been reported that the proton transfer occur and an ionized PIL can be obtained as long as the ΔpKa is at least 4 (e.g., see NPL 7). Dai et al. have been described that, based on an energy level diagram, thermal stability of the protic ionic liquid can be greatly improved by combining a strong acid with a strong base (e.g., see NPL 8). Watanabe et al. have reported that proton transferability and thermal stability of the protic ionic liquid greatly depend on ΔpKa, and, therefore, when DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) is used as the base, the ionic liquid is greatly improved in the thermal stability by using an acid having a pKa value so as to give the ΔpKa of 15 or more (e.g., see NPL 9). However, it has not been sufficiently discussed what level of ΔpKa is required for a certain application.